Combined Scanning Techniques Get Leg Up on Pulmonary Emboli

Action Points

Explain to interested patients that specialized CT scans are good at picking up the presence of serious or potentially fatal blood clots in the lungs, yet additional clinical assessment and imaging studies can boost the accuracy of the tests even further.

The chances of detecting pulmonary emboli may significantly improve when imaging of the leg veins and clinical assessment are superimposed on chest imaging studies, reported investigators in a multicenter trial.

When negative imaging studies conflict with clinical assessments indicating a high likelihood of embolism, clinicians would do well to trust their instincts and judgment and order additional studies to rule out the possibility that CT scans missed something, the investigators wrote in the June 1 issue of the New England Journal of Medicine.

Combining multidetector computed tomographic angiography of the chest with CT venography of the legs boosted the sensitivity rate to 90% from 83% for CT angiography alone, said the researchers in the Prospective Investigation of Pulmonary Embolism Diagnosis (PIOPED) II trial.

"This study suggests that chest CT angiogram for detecting dangerous blood clots in the lung is good, but sometimes it is not enough," said Paul D. Stein, M.D., Wayne State in Detroit and director of research education at St. Joseph Mercy Oakland Hospital here.

"We can more accurately detect or rule out pulmonary embolism by taking pictures of the leg veins in addition to pictures of the lung arteries," he and colleagues continued. "Our study spells out the strengths and weaknesses of chest CTs for diagnosing pulmonary embolism, and will help guide physicians on when more tests are needed."

The investigators also found that either CT angiography or venography was highly predictive of pulmonary embolism when combined with an objective clinical assessment (Wells Score). However, when imaging studies are negative for clots but patients still have a high Wells score, additional evaluation to rule out pulmonary embolism is warranted, the investigators cautioned.

"These data, along with those from recent outcome studies, support the use of multidetector CT angiography for suspected pulmonary embolism as a stand-alone imaging technique in most patients," wrote Arnaud Perrier, M.D., and Henri Bounameaux, M.D., of Geneva University in Switzerland, in an accompanying editorial.

The editorialists cautioned, however, that the CT venography does not appear to add enough clinical benefit to offset the additional radiation exposure involved.

The PIOPED II trial was a prospective, multicenter study designed to explore the accuracy of multidetector CT angiography either alone or in combination with venous-phase imaging (CT venography) for the diagnosis of acute pulmonary embolism.

The authors used a composite reference standard to confirm or rule out the diagnosis of pulmonary embolism in the study participants. The standard required either a ventilation-perfusion lung scan showing a high probability of pulmonary embolism in a patient with no history of pulmonary embolism, abnormal findings on pulmonary digital subtraction angiography, or abnormal findings on venous ultrasonography in a patient without previous deep venous thrombosis at that site, and non-diagnostic results on ventilation-perfusion scanning.

A total of 824 patients had a reference diagnosis and a completed CT study, but because of poor image quality the CT angiography was inconclusive in 51 patients, and these results were excluded from the analysis. Similarly, 87 patients were excluded from the combined angiography-venography analysis because of poor image quality.

After excluding the inconclusive studies, the investigators found that sensitivity of CT angiography of the chest alone was 83% (95% confidence interval, 76%-92%) and the specificity was 96% (95% CI 93%-97%). The likelihood ratio for a positive test was 19.6 (95% CI, 13.3 to 29.0), and for a negative test was 0.18 (95% CI, 0.13 to 0.24).

Among 737 patients for whom there were adequate CT angiography and venography studies, the sensitivity of the combined modalities for diagnosis of pulmonary embolism was 90% (95% CI, 84 to 93%), and the specificity was 95% (95 % CI, 92 to 96%).

The likelihood ratio for a positive test was 16.5 (95% CI, 11.6 to 23.5), and for a negative test was 0.11 (95% CI, 0.07 to 0.16). The positive predictive value was 85% (95 CI, 78 to 89 %), and the negative predictive value was 97% (95% CI, 94 to 97%).

When the clinical assessment was thrown into the mix, however, the predictive values of the imaging techniques varied significantly, the authors found.

"Among patients with a previous clinical assessment of high or intermediate probability of pulmonary embolism, the respective positive predictive values for pulmonary embolism were 96% (22 of 23 patients) and 92% (93 of 101 patients) for CT angiography," they wrote.

"Among patients with a low clinical probability of pulmonary embolism, 42% of the CT angiography readings were false positive. Similar positive predictive values were obtained for CT angiography-CT-venography," they added.

Among patients with a low clinical probability (i.e., low Wells score) the negative predictive value for CT angiography for excluding pulmonary embolism was 96%, and the negative predictive value for CT angiography and venography combined was 97%.

But among patients with a high clinical probability, 40% of results on CT angiography and 18 % of results on the combined modalities were false negative.

In their editorial, Dr. Perrier and Dr. Bounameaux caution that "clinicians should be wary of results that are discordant with their clinical judgment, especially in the rare case of a patient with a high likelihood of pulmonary embolism and normal findings on CT angiography. CT venography does not appear to improve the diagnostic yield of CT angiography enough to justify the additional irradiation."

The PIOPED II study was funded by the National Heart, Lung, and Blood Institute.

Reviewed by Zalman S. Agus, MD Emeritus Professor at the University of Pennsylvania School of Medicine